This invention relates to inverter controlled rotating field machines in general and more particularly to an improved method and apparatus for operating such machines at slow speed.
The use of a converter controlled rotating field machine is well known in the prior art. Typcially in such systems the AC voltage from the 3 phase line is rectified and provided through a DC link to a controlled inverter from which current is supplied to the rotating field AC machine. Typically the magnitude of the current is controlled in the DC link. Typical of such systems is that shown in U.S. application Ser. No. 63,073, a continuation of U.S. Ser. No. 237,572 based on German Offenlegunsshrift 1,941,312. A problem exists in such systems however in that only a discrete number of stator vector positions are possible. That is, depending on the number of control elements in the inverter and the number of windings in the machine, energization corresponding only to predetermined discrete positions of the stator current vector is possible i.e. for energization each pair of control devices in the converter, a specific vector results. In operation the vector is continuously stepped through 360.degree. to obtain rotation of the machine. At high speeds such stepping does not cause problems and the machine rotates smoothly. However, at low speeds, the discrete positions can cause undesirable harmonics in the torque.
Inverters do exist which can be commutated in both directions. Such an inverter with a center tap circuit and sequential phase quenching is disclosed in U.S. Pat. No. 3,733,543. However, the sequential phase quenching circuit disclosed therein has a major disadvantage in that the windings of the asynchronous machine are loaded with a d-c component and thus must be made accordingly larger.